This invention relates to optical fiber connector calibration and, more particularly to a tunable optical fiber connector for use in tunable calibrating jumper terminations.
In optical fiber communications, connectors for joining fiber segments at their ends, or for connecting optical fiber cables to active or passive devices, are an essential component of virtually any optical fiber system. The connector or connectors, in joining fiber ends, for example, has, as its primary function, the maintenance of the ends in a butting relationship such that the core of one of the fibers is axially aligned with the core of the other fiber so as to maximize light transmissions from one fiber to the other. Another goal is to minimize back reflections. Alignment of these small diameter fibers is extremely difficult to achieve, which is understandable when it is recognized that the mode field diameter MFR of, for example, a singlemode fiber is approximately nine (9) microns (0.009 mm). The MFR is slightly larger than the core diameter. Good alignment (low insertion loss) of the fiber ends is a function of the transverse offset, angular alignment, the width of the gap (if any) between the fiber ends, and the surface condition of the fiber ends, all of which, in turn, are inherent in the particular connector design. The connector must also provide stability and junction protection and thus it must minimize thermal and mechanical movement effects.
In the present day state of the art, there are numerous, different, connector designs in use for achieving low insertion loss and stability. In most of these designs, a pair of ferrules (one in each connector), each containing an optical fiber end, are butted together end to end and light travels across the junction. Zero insertion loss requires that the fibers in the ferrules be exactly aligned, a condition that, given the necessity of manufacturing tolerances and cost considerations, is virtually impossible to achieve, except by fortuitous accident. As a consequence, most connectors are designed to achieve a useful, preferably predictable, degree of alignment, some misalignment being acceptable.
Alignment variations between a pair of connectors are the result of the offset of the fiber core centerline from the ferrule centerline. This offset, which generally varies from connector to connector, is known as xe2x80x9ceccentricityxe2x80x9d, and is defined as the distance between the longitudinal centroidal axis of the ferrule at the end face thereof and the centroidal axis of the optical fiber core held within the ferrule passage and is made up of three vectors. It is often the case, generally, that the ferrule passage is not concentric with the outer cylindrical surface of the ferrule (vector I), which is the reference surface. Also, the optical fiber may not be centered within the ferrule passage (vector II whose magnitude is the diametrical difference divided by two) and, also, the fiber core may not be concentric with the outer surface of the fiber (vector III). Hence eccentricity can be the result of any one or all of the foregoing. The resultant eccentricity vector has two components, magnitude and direction. Where two connectors are interconnected, rotation of one of them will, where eccentricity is present, change the relative position of the fiber cores, with a consequent increase or decrease in the insertion loss of the connections. Where the magnitude of the eccentricities are approximately equal the direction component is governing, and relative rotation of the connectors until alignment is achieved will produce maximum coupling.
There are numerous arrangements in the prior art for xe2x80x9ctuningxe2x80x9d a connector, generally by rotation of its ferrule, to achieve an optimum direction of its eccentricity. One such arrangement is shown in U.S. Pat. No. 5,481,634 of Anderson et al., wherein the ferrule is held within a base member that maybe rotated to any of four rotational or eccentricity angular positions. In U.S. Pat. No. 4,738,507 of Palmquist there is shown a different arrangement and method for positioning two connectors relative to each other for minimum insertion loss or maximum coupling. The arrangements of these patents are examples of the efforts to achieve optimum reliable coupling, there being numerous other arrangements and methods.
In such arrangements for achieving optimum coupling with connectors having different magnitudes and directions of eccentricities, the tuning takes place, usually, if not always, prior to the final assembly of the connector. As a consequence, an installer in the field has no control over the degree of coupling, other than by trial and error. Further, tuning of the connector cannot be performed after production of the connector is completed. Thus tuning prior to final assembly of the conductor is a step in the production process.
In U.S. Pat. No. 6,287,018 of Andrews et al. there is shown a tunable optical fiber connector which can be tuned for optimum performance after the connector has been fully assembled, and, as a consequence, greatly reduces production costs, imparts greater reliability, and gives an installer in the field of measure of control of the connections being made.
The connector of that application has an enlarged barrel member, preferably hexagonal in shape, or alternatively has six slots about the periphery, thereby establishing six rotational positions for tuning the connector. A tuning test tool is provided for optimal tuning of the connector by means of a test jumper connector having a known eccentricity vector of predetermined magnitude and direction. The barrel of the connector under test is rotated to that one of the six positions that yields maximum signal transmission or minimum insertion loss. The arrangement, for optimum tuning, requires a test jumper of extreme accuracy, preferably having a magnitude of its eccentricity vector greater than the eccentricity magnitude of the connector being tuned, and either a 0xc2x0 or 180xc2x0 radial position. Such test jumpers are found within the laboratory, or by being one of a large number of jumpers of which only one or two may meet the desired vector requirements. Thus a usable test jumper is a rarity and, when found, is to be carefully preserved. Such test jumpers are, for these reasons, comparatively quite expensive. Thus a readily reproducible test jumper is a desirable tool.
The present invention is a tunable optical fiber connector for producing extremely accurate tuning jumpers, for example, which can be produced on demand, or on a production basis. The connector also makes possible ultra low-loss fiber connectors by the exceptionally accurate tuning thereof, and is also useful in polarization maintaining PM fiber applications.
The principles of the present invention are shown as embodied in an LC type connector for singlemode fibers, but it is to be understood that they are equally applicable to other types of connectors such as, for example, SC, FC, MU, and ST type connectors, as well as other fiber optic devices.
The connector of the invention which, for purposes of illustration of a preferred embodiment is a modified LC type connector as shown in U.S. Pat. No. 5,481,634 of Anderson et al., the disclosure of which is incorporated by reference herein, comprises a barrel-ferrule assembly for holding the end of an optical fiber extending axially therethrough and a housing for the assembly, a coil spring member contained within the housing surrounds the barrel, which is of tubular configuration and bears against an interior wall of the housing and an enlarged flange member on the barrel, thereby supplying forward bias to the barrel-ferrule assembly relative to the housing. As is shown in the aforementioned U.S. patent applications, the barrel-ferrule assembly, the enlarged flange member is hexagonal in shape and has a tapered or chamfered leading surface that may be slotted. The housing, in turn, has a hexagonally shaped cavity, which provides any of six rotational positions for the flange and a tapered seating surface for the tapered surface of the flange. The dimensions of the cavity are such that the hexagonal barrel flange floats within the hexagonal cavity, in the Anderson et al. arrangement and can rotate about xc2x112xc2x0, which diminishes the tuning accuracy. Additionally, the flange is affixed to the barrel, hence the barrel has only six positions, which are subject to the uncontrolled float.
In accordance with the present invention, the barrel of a connector comprises an elongated cylindrical member having, at its front end, a recess into which the fiber containing ferrule fits and is affixed thereto. The extreme end of the cylindrical member is in this illustrative embodiment slotted to accommodate an adjusting tool, such as the tuning wrench shown in the aforementioned U.S. Pat. No. 6,155,146 of Andrews et al. Immediately adjacent the slotted end is a cylindrical surface that ends in a flange, preferably integral with the cylindrical member. An enlarged tuning member such as a hexagonal tuning nut is bored to be a light press fit on the cylindrical surface and buts against the flange which functions as a locating stop. The front face of the tuning nut member is tapered in the manner disclosed in the aforementioned U.S. application and in U.S. Pat. No. 6,155,146 to fit within a tapered recess in the connector housing. By xe2x80x9clight press fitxe2x80x9d is meant a press fit sufficiently tight to prevent accidental movement of the two parts relative to each other, but which with application of torque permits relative rotary movement therebetween in tuning the connector. Further, in accordance with the invention, the flats on the tuning nut are enlarged to where there is barely a sliding fit of approximately 0.0005 inches to 0.001 inch clearance between the hexagonal tuning nut flat and the hexagonal bore flat within the connector housing. As will be explained in detail hereinafter, such reduction in clearance between the flats in the housing recess and the flats on the hex tuning nut produces a drastic reduction in angular float.
In tuning the connector terminating a fiber and with the barrel of the invention mounted therein, the tuning steps described in the aforementioned pending patent applications are in U.S. Pat. No. 6,155,146 are, basically, followed. Utilizing the tuning tool and the tuning wrench of those applications, a known jumper cable and connector is used to tune the connector of the present invention, by the method therein shown. Inasmuch as there are preferably six angular positions, for an ordinary connector there are 60xc2x0 positioned increments, which produces sufficiently acceptable tuning. However, for a jumper test cable and connector, as well as for PM fiber connectors, this is not nearly precise enough. Thus, in the tuning process, the closest angular position of the barrel in the connector is determined and then the barrel position is fine-tuned by using the slots in the front of the cylindrical member and the wrench, for example, to apply sufficient torque to overcome the light press fit to rotate the cylindrical member relative to the hexagonal nut, which is held in fixed position in the hexagonal recess in the connector housing, until the optimum position is reached. Because the float is very slight, as discussed hereinbefore, the optimum position can be obtained within very small tolerances. As will be apparent herein, means other than slots may be used for applying the required torque.
The principles and features of the present invention will be more readily understood from the following detailed description, read in conjunction with the accompanying drawings.